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variac/dc power supply repair

R

Ryan Underwood

Jan 1, 1970
0
Hi,
I just acquired a Lab-Volt model 189 bench supply, 1966 vintage. It has a
passthrough AC output, a variac output, and a variable DC output.
Unfortunately, the variable outputs do not work (0 voltage independent of
knob setting). The pass through output does work though.

The fuse is a screw-in type, black thumb screw reads "Fuse" and when it pulls
out, there is a loose metal cylinder around the end of the fuse. I have no
idea how to test this type of fuse or even what it is called.

Anyone have pointers where to get started on this thing? The construction
appears to be nothing more than a bunch of transformers and a knob. I'm not
familiar with these variable type supplies.

Thanks.
 
A

Arfa Daily

Jan 1, 1970
0
Ryan Underwood said:
Hi,
I just acquired a Lab-Volt model 189 bench supply, 1966 vintage. It has a
passthrough AC output, a variac output, and a variable DC output.
Unfortunately, the variable outputs do not work (0 voltage independent of
knob setting). The pass through output does work though.

The fuse is a screw-in type, black thumb screw reads "Fuse" and when it
pulls
out, there is a loose metal cylinder around the end of the fuse. I have
no
idea how to test this type of fuse or even what it is called.

Anyone have pointers where to get started on this thing? The construction
appears to be nothing more than a bunch of transformers and a knob. I'm
not
familiar with these variable type supplies.

Thanks.
The fuse is easily enough tested using a simple ohm meter, or by
substitution, but to be honest, if you do not have sufficient knowledge to
be able to troubleshoot a simple power supply of this vintage, I really
would not recommend that you start poking around inside, as there are
dangerous voltages present in any mains power supply. Sorry to be so
negative about this, but I'm only thinking of your safety ...

Arfa
 
R

Ross Herbert

Jan 1, 1970
0
Hi,
I just acquired a Lab-Volt model 189 bench supply, 1966 vintage. It has a
passthrough AC output, a variac output, and a variable DC output.
Unfortunately, the variable outputs do not work (0 voltage independent of
knob setting). The pass through output does work though.

The fuse is a screw-in type, black thumb screw reads "Fuse" and when it pulls
out, there is a loose metal cylinder around the end of the fuse. I have no
idea how to test this type of fuse or even what it is called.

Anyone have pointers where to get started on this thing? The construction
appears to be nothing more than a bunch of transformers and a knob. I'm not
familiar with these variable type supplies.

Thanks.

If it is similar to this model it shouldn't be too difficult to find
out what is wrong with it. Looks pretty basic but I haven't actually
seen one. BTW, if you can't determine whether a fuse is blown or not
then you shouldn't be playing around inside it.
http://it.stlawu.edu/~physics/labs/common/img/labvolt.jpg
 
S

Sam Goldwasser

Jan 1, 1970
0
Ryan Underwood said:
Hi,
I just acquired a Lab-Volt model 189 bench supply, 1966 vintage. It has a
passthrough AC output, a variac output, and a variable DC output.
Unfortunately, the variable outputs do not work (0 voltage independent of
knob setting). The pass through output does work though.

The fuse is a screw-in type, black thumb screw reads "Fuse" and when it pulls
out, there is a loose metal cylinder around the end of the fuse. I have no
idea how to test this type of fuse or even what it is called.

Anyone have pointers where to get started on this thing? The construction
appears to be nothing more than a bunch of transformers and a knob. I'm not
familiar with these variable type supplies.

Agreed with the other postings, especially on the SAFETY issues.

Most likely problme is a blown fuse (there might be nother one inside),
next most likely would be a bad connection or short somewhere, next most likely
might be a bad carbon brush on the Variac. There really isn't much
else to go wrong, though a shorted rectifier might be the cause of the
blown fuse.

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line. Or, you can contact me via the Feedback Form in the FAQs.
 
Ross said:
If it is similar to this model it shouldn't be too difficult to find
out what is wrong with it. Looks pretty basic but I haven't actually
seen one. BTW, if you can't determine whether a fuse is blown or not
then you shouldn't be playing around inside it.
http://it.stlawu.edu/~physics/labs/common/img/labvolt.jpg

Yeah, that's the same style of fuse. I just don't know how to test
that type of fuse. I am only familiar with repairing PC power supplies
and the glass/filament type fuses that they use.
 
S

Sam Goldwasser

Jan 1, 1970
0
Yeah, that's the same style of fuse. I just don't know how to test
that type of fuse. I am only familiar with repairing PC power supplies
and the glass/filament type fuses that they use.

What do you mean by "that type of fuse". What type is it?

In any case, you test that fuse like any other, with a multimeter on
ohms or continuity. It should be read as close to 0 ohms.

Respectfully, if you don't even know how to test a fuse, what are you
going to use this power supply for, and do it safely?

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line. Or, you can contact me via the Feedback Form in the FAQs.
 
I am fully aware of how to test a fuse, given that I can find the
terminals of the fuse, which is not usually an unreasonable assumption.
If you look at the picture, it is a type of fuse that screws in. It
is not like the "edison" type screw-in fuses. There is kind of a
"rivet" in the end of it that, when removed, retains a metal cylinder
that hangs loosely over the rivet. I can't figure out how the fuse is
supposed to work, because when I've removed it, it physically becomes
one big short because of the loose cylindrical part.

I am fully aware of the hazards, but from the replies, you'd think I
was prodding around in it with a kitchen knife while plugged in. All I
am trying to do is test this fuse, and if disassembly is required, I
have no plans to do that with it plugged into the wall or with an
energized filter cap.
 
S

Sam Goldwasser

Jan 1, 1970
0
I am fully aware of how to test a fuse, given that I can find the
terminals of the fuse, which is not usually an unreasonable assumption.
If you look at the picture, it is a type of fuse that screws in. It
is not like the "edison" type screw-in fuses. There is kind of a
"rivet" in the end of it that, when removed, retains a metal cylinder
that hangs loosely over the rivet. I can't figure out how the fuse is
supposed to work, because when I've removed it, it physically becomes
one big short because of the loose cylindrical part.

It sounds to me like the fuse is missing.

The fuse that goes in there is probably 1-1/4" long by 1/4" in diameter.
I am fully aware of the hazards, but from the replies, you'd think I
was prodding around in it with a kitchen knife while plugged in. All I
am trying to do is test this fuse, and if disassembly is required, I
have no plans to do that with it plugged into the wall or with an
energized filter cap.

When someone asks how to test a fuse, it is no wonder that people are
questioning your experience.

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line. Or, you can contact me via the Feedback Form in the FAQs.
 
R

Ross Herbert

Jan 1, 1970
0
I am fully aware of how to test a fuse, given that I can find the
terminals of the fuse, which is not usually an unreasonable assumption.
If you look at the picture, it is a type of fuse that screws in. It
is not like the "edison" type screw-in fuses. There is kind of a
"rivet" in the end of it that, when removed, retains a metal cylinder
that hangs loosely over the rivet. I can't figure out how the fuse is
supposed to work, because when I've removed it, it physically becomes
one big short because of the loose cylindrical part.

I am fully aware of the hazards, but from the replies, you'd think I
was prodding around in it with a kitchen knife while plugged in. All I
am trying to do is test this fuse, and if disassembly is required, I
have no plans to do that with it plugged into the wall or with an
energized filter cap.

It may simply be that the fuse holder is broken...

If the fuse is a screw cap then you simply unscrew the cap
anti-clockwise. The fuse will most likely be a 3AG (1-1/4" x 1/4"
tubular glass). Usually the screw cap has a flat spring retainer to
hold the new fuse when inserting it but these sometimes break or go
missing butthis doesn't usually affect the function of the fuse
holder. At the far end of the fuse holder the rear terminal is spring
loaded so that pressure is applied to both ends of the fuse when the
cap is screwed home. You can test to see if the spring exerts pressure
by pushing a screwdriver blade down inside the fuse holder.

I have found these fuseholders sometimes go open circuit usually
through a terminal or phenolic body breakage internally. The only way
to test the fuse holder is to disconnect (unsolder) one of the wires
connecting to it (the far end one is easiest) and with a known good
fuse inserted measure across both terminals of the fuse holder with
your DMM. Naturally, you must ensure that no power is applied while
doing any of this.
 
J

James Sweet

Jan 1, 1970
0
I am fully aware of how to test a fuse, given that I can find the
terminals of the fuse, which is not usually an unreasonable assumption.
If you look at the picture, it is a type of fuse that screws in. It
is not like the "edison" type screw-in fuses. There is kind of a
"rivet" in the end of it that, when removed, retains a metal cylinder
that hangs loosely over the rivet. I can't figure out how the fuse is
supposed to work, because when I've removed it, it physically becomes
one big short because of the loose cylindrical part.

Are you sure there's even a fuse in there? Almost sounds to me like an
empty holder.
 
Sam, save the flames for someone more deserving, please. I said that I
have never seen THIS TYPE of fuse before, because I mistook what is
apparently a broken fuse holder for the fuse itself. That is not
equivalent to asking "how do I test a fuse?". As for "questioning my
experience", go right ahead, I'm not going to get into a pissing match,
but it is usually less effort to simply answer the question as posed.

I openly admit that I rarely get to deal with nice test equipment. And
I don't see where that means that I should simply accept that it is
broken and not attempt to learn about it - just as I learned about
microwaves, televisions, monitors, amplifiers or any other piece of
electronics that could be dangerous if one does not employ common sense.
 
Yes, it appears to be an empty, broken fuse holder. When I first
unscrewed it, I was confused and thought it was similar to the mains
fuses that screw into the panel. A step in the right direction, thanks!
 
J

James Sweet

Jan 1, 1970
0
Yes, it appears to be an empty, broken fuse holder. When I first
unscrewed it, I was confused and thought it was similar to the mains
fuses that screw into the panel. A step in the right direction, thanks!


It doesn't even sound broken to me, often the fuse is all that holds the
bottom contact down away from the cap, with no fuse at all installed all
bets are off.
 
R

Ross Herbert

Jan 1, 1970
0
because I mistook what is apparently a broken fuse holder
for the fuse itself.

So, are you now sure that the problem was caused by a broken fuse
holder in the first place?
 
J

James Sweet

Jan 1, 1970
0
Ross said:
So, are you now sure that the problem was caused by a broken fuse
holder in the first place?


Well there's no sense in arguing about it or belittling one another, I
would however strongly recommend doing some research on the subject
before mucking around with it too much. I do agree that a person should
be able to clearly identify just about any type of fuse or fuse holder
they come across before poking around in anything mains powered.
 
I got a service diagram from Lab-Volt after much consternation. Looks
like the holder is supposed to be like that, when the holder screws in
it both tightens up the "link" to the fuse as well as completes the
connection. So the holder may not be broken, I'll just see if it blows
another fuse before proceeding. Thanks for the helpful replies.
 
R

Ross Herbert

Jan 1, 1970
0
I got a service diagram from Lab-Volt after much consternation. Looks
like the holder is supposed to be like that, when the holder screws in
it both tightens up the "link" to the fuse as well as completes the
connection. So the holder may not be broken, I'll just see if it blows
another fuse before proceeding. Thanks for the helpful replies.


You are a worry... this is the first time you have even mentioned that
the unit is "blowing fuses". Your previous posts inferred that the
problem was with continuity related to this fuse, but this new
information puts a whole new complexion on things. The very fact that
the fuse is blowing infers there is nothing wrong with the fuse holder
or the fuse. Assuming that you are using a properly rated delay fuse
(slo-blo, or time delay fuse usually with a 'T' marking), and it is
still 'blowing', then the fuse and holder are fine. The most likely
problem is that there is some component following the fuse which is
overloading it and causing it to do its intended job.

Have you even opened the case to see if there is any obvious sign of a
short circuit anywhere? Perhaps I shouldn't ask that if your attempt
to repair the unit up till now is an indication of your knowledge.
 
S

Smitty Two

Jan 1, 1970
0
Ross Herbert said:
You are a worry... this is the first time you have even mentioned that
the unit is "blowing fuses". Your previous posts inferred that the
problem was with continuity related to this fuse, but this new
information puts a whole new complexion on things. The very fact that
the fuse is blowing infers there is nothing wrong with the fuse holder
or the fuse. Assuming that you are using a properly rated delay fuse
(slo-blo, or time delay fuse usually with a 'T' marking), and it is
still 'blowing', then the fuse and holder are fine. The most likely
problem is that there is some component following the fuse which is
overloading it and causing it to do its intended job.

Have you even opened the case to see if there is any obvious sign of a
short circuit anywhere? Perhaps I shouldn't ask that if your attempt
to repair the unit up till now is an indication of your knowledge.

The OP's claims to the contrary, it seems to me that he doesn't know a
fuse from a fuse holder. Fine, ignorance is OK, and if he's a novice, I
have no problem with that. But, his complete inability to see what he
sees and communicate it clearly is exceptionally frustrating. One
picture link would help us help him.
 
S

Sam Goldwasser

Jan 1, 1970
0
Smitty Two said:
The OP's claims to the contrary, it seems to me that he doesn't know a
fuse from a fuse holder. Fine, ignorance is OK, and if he's a novice, I
have no problem with that. But, his complete inability to see what he
sees and communicate it clearly is exceptionally frustrating. One
picture link would help us help him.

It's of more concern that if he doesn't know a fuse from a fuse holder,
he may not know the difference between a live wire and ground and may end
up dead.

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line. Or, you can contact me via the Feedback Form in the FAQs.
 
I know the difference between a fuse and a fuse holder in terms of
their function. However, I have not seen every fuse holder nor every
fuse. I jumped to a conclusion at the very beginning when I thought I
was looking at some strange, obsolete type of *fuse* when I was looking
at a *holder* of type which is apparently common and well known in test
equipment. How should I know? That's why I asked. (You know, the
holder says "Slo Blo" on it too.)

To reply to the GP, with this knowledge of what is a fuse and what is a
holder straightened out several posts ago, I concluded that it was
given to me without a fuse. Isn't it a reasonable presumption then
that it previously blew a fuse? What other reason would one have for
removing a fuse? No, there are no obvious short circuits in that there
are no loose wires. Since the presumption was that a fuse blew, that
was the first thing to check. But I can't tell whether it will blow
another fuse without putting another fuse in and powering it up, can I?

I don't see what has been so difficult to understand about what I have
posted. The largest part of the problem is that respondents would
rather jump to conclusions about my mettle than ask for clarification
if what I have posted is ambiguous or seems wrong. I try to anticipate
replies when I post, but it is inescapable that descriptions will be
lacking at times. Furthermore, I have visited this project
approximately three times; since it is at home and I work all day my
opportunities are limited.

My experience is limited to consumer electronics component-swapping
(including fuses). That's why I haven't seen this type of holder
before - fuse holders in consumer equipment are almost invariably the
flex metal type. I can take the appropriate precautions vs becoming a
crispy critter when working on a monitor, a PC power supply, or a
microwave, so why do I feel like I need a disclaimer at the end of
every post I make about this power supply?

I realize you get what you pay for in terms of soliciting advice, but I
recall this group being a lot less hostile 5 years ago.
 
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